Coaxial cable with helical insulating spacer



Aug. 25, 1964 K. H. HAHNE 3,146,297

COAXIAL CABLE WITH HELICAL INSULATING SPACER Filed Feb. 17, 1961 2% IN VEN TOR.

K AIZL HEINZ HAHNE F 16. 3

United States Patent 3,146,297 CQAXIAL CABLE WHTH HELlCAL INSULATING SPACER Karl Heinz Hahne, Junlrersdorf, near Cologne, Germany, assignor to Felten dz Guilleaurne Carlswerk A.G., Cologne-Mulheirn, Germany, a corporation of Germany Filed Feb. 17, 1961, Ser. No. 109,469 Claims priority, application Germany Feb. 25, 1960 2 Claims. (Cl. 174-.29)

The present invention relates to coaxial cable and more particularly to coaxial cable having a helical insulating spacer and a helical outer conductor.

Coaxial cables for the electric conduction of high-frequency alternating currents generally consist of a metal inner conductor, insulating spacers arranged concentrically about the inner conductor, and a metal tubular outer conductor concentrically around, and supported by, the spacers. This type of cable should have low transmission attenuation, suificient flexibility to be curved, in placement and in transportation, without breaking the cable or lowering its transmission quality, and substantially equal transmission characteristics for equal lengths of the cable.

An insulating helix, rather than individual spacing plates, is preferred for coaxial cable because of its high dielectric insulating properties, uniformity of spacing, continuous productivity, and low cost of manufacture. The helical spacer, which is preferably of a plastic material, is injection-molded directely onto the inner conductor by automatic continuous machinery. The helix may also be constructed by winding either a single or a plurality of tapes in helical fashion about the inner conductor.

While the insulating helix is flexible, the outer tubular conductor may break under bending. A number of types of cables have outer conductors which bend satisfactorily. In one type the outer conductor is a braid of metal threads, but it is inferior in transmission quality to a cable having a tubular outer conductor with solid side walls. Another type has a tubular outer conductor with solid side walls made of a relatively flexible metal such as lead. However, lead conducts electricity poorly especially at high frequencies, at which it is the skin or surface effect of the conductors which is of primary importance. In other types the outer tubular conductor has a corrugated solid side wall, with either the corrugations arranged in a series of doughnut-shaped metal rings stacked one upon the other or in a helical form. In the helical form the corrugations follow a symmetrical pattern, both the troughs and crests of the corrugation being curves which are symmetrical to each other. In those cables having a helically corrugated outer conductor and a helical insulating spacer the helixes are laid in opposite directions to each other to provide bendability. With that construction the crests of the outer conductor and the crests of the spacer are positioned at random relative to each other. This random positioning results in unequal transmission characteristics for equal lengths of the cable and, as the spacer has a different dielectric constant than air, attenuation by reflection of certain high frequencies.

It is an objective of the present invention to provide a coaxial cable which may be curved in transportation and in placement Without adversely affecting its transmission characteristics.

It is a further objective to provide such a cable in which the transmission characteristics are susbtantially uniform over equal lengths of the cable.

Another objective is that the insulating spacer of the cable does not attenuate high frequency waves by reflection.

Another objective is that the cable be convenient to manufacture by automatic and continuous machinery.

In accordance with the present invention a coaxial cable is provided having an outer conductor formed with a corrugation spiralling around the inner conductor, and an insulating helix of a high dielectric material spirally wound about the inner conductor and serving as a spacer between the outer conductor and the inner conductor. The insulating helix is arranged so that the outer periphery of its turns is within the crest of the spiral corrugation of the outer conductor, the turns of the corrugation being separated by a valley portion of the outer conductor which spirals around the inner conductor between the turns of the insulating helix. By suitable proportioning of the insulating helical spacer and the dielectric constant of the spacer, the greater distance between inner and outer conductors at the crest of the corrugation of the outer conductor will electrically offset the lower insulation of the air space and the closer distance between the conductors at the directly opposite side of the cable where the valley portion of the outer conductor is located.

These and other objects and aspects of the invention will be apparent from the following description of a specific embodiment and by reference to the accompanying drawings wherein:

FIG. 1 is a cross-sectional side View of a coaxial cable of the present invention showing the top portion of the inner conductor in half elevation;

FIG. 2 is a cross-sectional partial side view enlarged with respect to the view of FIG. 1, showing crest and valley portions of the outer conductor and a preferred tool to form that conductor; and

FIG. 3 is a diagram of the same tool as that shown'in FIG. 2 illustrating the mathematical relationships involved in the helix of the outer conductor.

In FIG. 1 inner conductor 9 is a cylindrical electrically conducting metal member which may be solid but is shown as hollow. Electrically insulating spacer 10 in the form of a helix is secured concentrically around inner conductor 9 and is surrounded by outer conductor 11, the latter being an electrically conducting tube corrugated in the form of a helix and having a solid tubular wall.

The impedance of a coaxial cable is determined by the ratio of its inductance to its capacitance. The inductance is substantially the same along the length of the cable, but the capacitance varies, increasing with the insulating properties of the spacer and decreasing with the distance between conductors. For example, the insulating property of a plastic helix is approximately 2.5 times that of air, i.e., the plastic material has a dielectric constant of 2.5. In the coaxial cable of this invention, by a proper selection of the dome height of the crest b of the corrugation of the outer conductor and its width V (FIG. 3), taking into account the dielectric constant of the particular material to be used in the helical spacer, com pensation may be attained so that the capacitance along the cable is equal. This equality is attained because in all cross-sections taken perpendicular to the axis of the cable, there is uniformity in the amount of dielectric spacing material and air and in the spacing of the outer conductor 11 from the inner conductor 9. Phrased in another way, on a diameter taken perpendicular to the axis of the cable the outer periphery of the spacer (and the crest of the spiral corrugation of the outer conductor) is exactly opposite to, and electrically balanced by, an air gap and the valley of the outer conductor.

As the spiral outer periphery of the insulating helix It is held along its entire length within the crest of the spiral corrugation of the outer conductor, any mechanical displacements which occur due to the different coefficients of expansion of the outer conductor and the insulating helix are taken up by changes in the shape of the outer periphery of the insulating helix. The outer conductor and the insulating helix do not move in reiationship to one another, so that the electrical properties of the coaxial cable remain constant.

Preferably, the outer edge of the insulating helix Ill is curved prior to forming the outer conductor 11 about it, so that it conforms to the shape of the inner wall of the spiral crest of the outer conductor.

A further advantage of this construction occurs when the coaxial cable is used in a vertical position, for example, as a feed to a high frequency antenna. In these cases, a clamp is preferably utilized whose internal undulation profile is the same as that of the corrugation of the outer conductor. With this type of clamp, due to the construction of the cable, both the outer conductor and the insulating helix are clamped.

The outer conductor llll is shown in greater detail in FIG. 2 in which the wall thickness of the outer conductor is designated by t. The wall is bent by a forming tool 12., the tool preferably being in the form of a helix. The wide fiat portion 2 of tool 12 together with its curved fillet 3 compresses the tubular wall of the outer conductor to form a valley i. Portion 6, the crest of the spiral corrugation, is not compressed by the tool but remains in recess 5 of tool 12. Since portion 6 is not under pressure, it is free to shape itself and assumes a dome having a semi-circular cross-section. The dimensions of the tool and of the outer conductor are preferably given by certain relationships. For the formulae for these relation ships, an imaginary center line 8 is drawn in the center of the wall of the outer conductor. The width of the recess portion of forming tool 12 is designated e The width 2 is less, by the thickness t of the outer conductor, than the actual width e The radius r of the forming tool 12, taken to the imaginary line 8 or 8a, is greater than the radius r to the outer surface of the crest. In FIG. 3, b is the height of the dome of the crest of the outer conductor, R is the radius of the dome, V is the diameter of the arched circle forming that dome, and V is the distance across the base of the dome from the ends of fillet portions 3.

It has been found that the preferred optimum relationships are that the height of the dome of the crest b is equal to its radius R. 0f greater importance for strength and bendability, however, is that the fillet radius r be substantially equal to the dome radius R, so that the metal between forming the inflection zone between the vaileys and the crest of the corrugation is not overstressed. The length of this transitional portion formed by fillet 3 is preferably V=V The valleys 4 are preferably flat rather than curved. The valleys and crests preferably follow the following equations:

The maximum bendability of the coaxial cable is generally determined by the bendability of its outer conductor. The inner conductor is of relatively small diameter and therefore more readily bent while the intermediate insulating spacer is usually of a plastic or other flexible material.

When the outer conductor is bent to its maximum, for example, when the coaxial cable is wound upon a shipping drum, the top part of the cable (facing away from the axis of the drum) is stretched and the bottom portion is compressed.

In FIG. 3, V designates the crest and its oppositely curving inflection zone formed by fillet 3. 1-V, where V is less than 1, is the length of the cylindrical part (the flat valley portion) where the measurement of the uniform corrugation is set, in this specification, equal to a pitch of P. The pitch is the distance, along a longitudinal line, from a point at the center of one valley to a point in the center of the next valley. This distance of P will be modified by the actual measurement which will give a multiplication factor for the pitch of h. In the case of extreme stretching of the corrugation, the crests at the top of the bent cable tend to be flattened out so that the dimension from valley to valley is lengthened. The relative change in length Will now be equal to:

which is equal to a relative lengthening of the longitudinal element of the outer conductor in the stretched zone of letting the ratio of the diameter of bend D to the cable diameter d be equal to the bending factor k:

x 4b lV V For the optimum corrugation shape, in which the circle of curvature of the peak point of the crest is a part of a semicircle over the extension of the cylindrical part, the bending factor is:

k= 1 V 45 The optimum shape of the outer conductor is not attainable. However, it is possible to make V smaller in order to maximize the bendability of the cable or to make the height b of the dome smaller to save material. The limitations on the amounts by which the length V or the height b may be altered are that the fillet radius r should be equal to the dome radius R, resulting in the relationship:

V 2 cos are tan 56 It should be understood that the present disclosure is for the purpose of illustration only and that this invention includes all modifications and equivalents which fall within the scope of the appended claims.

I claim:

1. In a coaxial cable, the-combination of an electrically conductive bare inner conductor of circular cross-section, an electrically insulating spacer of substantially rectangular cross-section spiralling around the bare inner conductor with the longer dimension of said cross-section extending substantially radially of the inner conductor, the spiral spacer forming a helix having its turns spaced along said inner conductor with the inner edge of the turns engaging the bare inner conductor around the periphery thereof to hold said inner conductor against radial displacement with in the turns, and a solid-walled tubular electrically conductive outer conductor surrounding the bare inner conductor in spaced relation thereto and having an outwardly pro truding corrugation spiralling around said inner conductor in spaced turns, the outer conductor also having a val ley portion spiralling around said inner conductor in spaced relation thereto and in spaced turns which separate said corrugation turns and which are located between said turns of the spiral spacer, said corrugation having a crest portion which is located at a substantially greater radius from the inner conductor than is said valley portion and which is dome-shaped in longitudinal section of the outer conductor, whereby the inner surface of said crest portion is concave, said turns of the spiral spacer having their outer edge located entirely in said corrugation and seated against said concave inner surface of the crest portion to hold said inner and outer conductors in fixed spaced relation to each other.

2. The combination defined in claim 1, in which said valley portion is flat in longitudinal cross-section and forms along the length of the cable a spiral wall of uniform radius with respect to the axis of the cable.

References Cited in the file of this patent UNITED STATES PATENTS 2,599,857 Mildner June 10, 1952 2,852,597 Raydt et al Sept. 16, 1958 FOREIGN PATENTS 764,175 France May 16, 1934 170,205 Switzerland Sept. 17, 1934 728,421 Germany Nov. 26, 1942 

1. IN A COAXIAL CABLE, THE COMBINATION OF AN ELECTRICALLY CONDUCTIVE BAR INNER CONDUCTOR OF CIRCULAR CROSS-SECTION, AN ELECTRICALLY INSULATING SPACER OF SUBSTANTIALLY RECTANGULAR CROSS-SECTION SPIRALLING AROUND THE BARE INNER CONDUCTOR WITH THE LONGER DIMENSION OF SAID CROSS-SECTION EXTENDING SUBSTANTIALLY RADIALLY OF THE INNER CONDUCTOR, THE SPIRAL SPACER FORMING A HELIX HAVING ITS TURNS SPACED ALONG SAID INNER CONDUCTOR WITH THE INNER EDGE OF THE TURNS ENGAGING THE BARE INNER CONDUCTOR AROUND THE PERIPHERY THEREOF TO HOLD SAID INNER CONDUCTOR AGAINST RADIAL DISPLACEMENT WITHIN THE TURNS, AND A SOLID-WALLED TUBULAR ELECTRICALLY CONDUCTIVE OUTER CONDUCTOR SURROUNDING THE BARE INNER CONDUCTOR IN SPACED RELATION THERETO AND HAVING AN OUTWARDLY PROTRUDING CORRUGATION SPIRALLING AROUND SAID INNER CONDUCTOR IN SPACED TURNS, THE OUTER CONDUCTOR ALSO HAVING A VALLEY PORTION SPIRALLING AROUND SAID INNER CONDUCTOR IN SPACED RELATION THERETO AND IN SPACED TURNS WHICH SEPARATE SAID CORRUGATION TURNS AND WHICH ARE LOCATED BETWEEN SAID TURNS OF THE SPIRAL SPACER, SAID CORRUGATION HAVING A CREST PORTION WHICH IS LOCATED AT A SUBSTANTIALLY GREATER RADIUS FROM THE INNER CONDUCTOR THAN IS SAID VALLEY PORTION AND WHICH IS DOME-SHAPED IN LONGITUDINAL SECTION OF THE OUTER CONDUCTOR, WHEREBY THE INNER SURFACE OF SAID CREST PORTION IS CONCAVE, SAID TURNS OF THE SPIRAL SPACER HAVING THEIR OUTER EDGE LOCATED ENTIRELY IN SAID CORRUGATION AND SEATED AGAINST SAID CONCAVE INNER SURFACE OF THE CREST PORTION TO HOLD SAID INNER AND OUTER CONDUCTORS IN FIXED SPACED RELATION TO EACH OTHER. 